For example, a tandem color laser printer is provided with a sheet conveyor belt or an intermediate transfer belt. A color laser printer provided with the sheet conveyor belt adopts a so-called direct transfer method. In this direct transfer method, during conveyance of a sheet by the sheet conveyor belt, toner images of respective colors of yellow, magenta, cyan and black are sequentially superimposed on and transferred to the sheet. Meanwhile, a color laser printer provided with the intermediate transfer belt adopts a so-called indirect transfer method. In this indirect transfer method, after toner images of respective colors of yellow, magenta, cyan and black are sequentially superimposed on and transferred to the intermediate transfer belt, a toner image on the intermediate transfer belt is transferred to a sheet at a time.
During the transfer of a toner image of each color, driving of the belts is controlled in a feedback manner such that the speed (traveling speed) of the sheet conveyor belt or the intermediate transfer belt is detected, and the speed of both belts is maintained at constant speed on the basis of the detected speed. If there is irregularity in the speed of the belts, deviation is caused in the transfer position of a toner image of each color on the sheet or the intermediate transfer belt. Therefore, the feedback control requires high precision.
As a technique of detecting the speed of each belt, for example, it is considered that an encoder is attached to a belt-supporting roller, the rotational speed of the roller is obtained from output pulses from the encoder, and the speed of the belt is calculated (estimated) on the basis of the obtained rotational speed. However, because the belt is an elastic body, the speed of the belt changes due to micro-vibration caused during traveling of the belt, even when the roller rotates at constant rotational speed. Accordingly, since the speed of the belt calculated from the rotational speed of the roller is not necessarily equal to an actual speed of the belt, the rotational speed cannot be used to control driving of the belt.
Thus, for example, JP-A-2004-198624 suggests providing an intermediate transfer belt with a scale in which a number of scale slits are formed at regular intervals, providing a sensor that outputs signals in response to the detection of the scale slits, at a position where the scale can be read, and calculating the speed of the intermediate transfer belt on the basis of the interval (interval from a previous output signal to the next output signal) of output signals from the sensor during driving of the intermediate transfer belt.